Rotor blades of wind turbine are exposed to large dynamic mechanical loads in particular when the wind turbine is operated in turbulent wind conditions or in conditions of flow distortion, e.g. high wind shear. Therefore, the rotor blades of wind turbines and the corresponding supporting structures have been dimensioned such as to be able to withstand all the mechanical loads that could occur under all conditions to which the wind turbine is certified. However, in case of extreme wind conditions the mechanical load acting on the rotor blades of the wind turbine must be reduced in order to avoid any damage of the wind turbine.
The risk for damaging a rotor blade strongly depends on the fatigue the blade is currently suffering and on the fatigue life time, which the rotor blade has accumulated so far during its previous operation. To be able to calculate the fatigue for instance in the root of the rotor blade, it is known to measure the deflection of the rotor blade in order to find out the current mechanical load the rotor blade is exposed to. If the rotor blade has a large deflection the fatigue load in the root is relatively big. The rotor blade deflection can be measured by means of a position sensor which is installed in the blade. With such a rotor blade sensor it is possible to individually regulate the mechanical load acting on the rotor blade for instance by pitching the respective blade. Thereby, a so called “cyclic pitching” can be applied. Another possibility for reducing high mechanical loads acting on rotor blades is to curtail the operation of the wind turbine such that less electrical power is generated. This can be realized for instance by pitching all rotor blades.
In this respect the term “cyclic pitching” refers to a method wherein the blade pitch angle setting of each rotor blade is changed during each revolution. More details about “cyclic pitching” can be found for instance in U.S. Pat. No. 4,298,313.
The current mechanical load acting on a rotor blade can be measured for instance by means of a strain gauge, which is used for measuring strains within a selected portion of a rotor blade. However, continuing load cycles of rotor blades cause a big wear of such strain gauges resulting in a reduced lifetime of the respective strain gauges. Another disadvantage of strain gauges is that they tend to drift over time and due to temperature changes. Furthermore, maintenance of strain gauges is often tedious when the respective strain sensor is located in an outer radial portion of the rotor blade. Further, when mounting a strain gauge at an outer radial rotor blade portion, for contacting the strain gauge electrical wires are necessary which run on or within the longitudinal extension of the blade and which make the rotor blade vulnerable to lighting strikes.
U.S. Pat. No. 4,297,076 discloses a wind turbine in which the tip portions of the rotor blades are variable in pitch and are cyclically varied in pitch in order to control the yaw angle of the rotor of the wind turbine and in order to relieve bending moments on the rotor blades. Further, the rotor blades are collectively varied in pitch in order to relieve bending moments on the blades and in order to maximize the power output of the turbine at a selected constant rotor speed.
U.S. Pat. No. 7,246,991 B2 discloses a wind turbine with a sensor that measures the out-of-plane deflection of the rotor blades and a controller that uses the signal from the sensor to determine the risk of a tower strike. The sensor can include strain gauges or accelerometers mounted on the rotor blades or it can include a fixed sensor mounted on the side of the tower.
There may be a need for improving deflection measurements for wind turbine rotor blades.